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1528 R. P. Daanen et al.: Rapid movement of frozen debris-lobes 1 Fig. 7. Comparison 2 Figure of the 7.Comparison termini positions of of the FDL-A termini andpositions FDL-B between of FDL-A 1955 and 2008 FDL-B frombetween high-resolution 1955 and aerial2008 and satellite based remotely sensed imagery. Two drainage channels in front of the lobe drain FDL-A and are not part of the moving body. The white arrow near the terminus 3 from of FDL-A high-resolution indicates theaerial repeatand photo satellite camerabased locationremotely illustratedsensed in Fig. imagery. 10. Two drainage channels Percent Passing 100 90 4 5 in front of the lobe drain FDL-A and are not part of the moving body. The white arrow near the terminus of FDL-A indicates the repeat photo camera 4.4 location Remote-sensing illustrated and in figure terrain 10. analysis FL-1 FL-2 80 FDL-A is elongate, varies in width from 140 to 260 m and has a length of 1200 m beyond its source area (Fig. 3). 70 Based on the IfSAR DEM, the height of the front of this frozen debris-lobe above the surrounding area is 20 m, ris- 60 ing to 25 m approximately 275 m behind (upslope) from the lobe front, and decreasing again to 10 to 18 m approx- 50 imately 500 m behind the lobe front. This indicates that 40 debris movement is episodic or pulse like, with periods of faster movement alternating with periods of stagnation and 30 buildup. Such movement pulses likely are controlled by 20 slope, ground temperature, liquid water content and debris supply. The slope of the surrounding area north and south of 10 the frozen debris-lobe front ranges from 5 0 100 10 1 0.1 Grain Size (mm) 0.01 0.001 33 Grain size Fig. distribution 8. Grain size of two distribution sediment of two samples sediment taken samples from taken the from top portion of the front L-A. the top portion of the front lobe of FDL-A. ◦ to 17.5◦ (Fig. 3). While the surface of the frozen debris-lobe has roughly the same slope along its long axis, multiple areas along the lobe front as well as its sides have slopes exceeding 30◦ , which agrees with field observations of steep and several metre high scarps. The entire watershed contributing to FDL-A is about 98 ha, one of the largest watersheds on this mountain slope (Fig. 2a). The lobe front, with its approximately 20-m height and 170-m width, advances a debris volume of about 34 m3 day−1 (12 410 m3yr−1 ) at the current movement rate of 1 cm day−1 observed with ground measurements between Nat. Hazards Earth Syst. Sci., 12, 1521–1537, 2012 www.nat-hazards-earth-syst-sci.net/12/1521/2012/
R. P. Daanen et al.: Rapid movement of frozen debris-lobes 1529 1 2 Figure 9.Tree rings with compression wood direction shifts. The tree was harvested in 2008. Fig. 9. Tree rings with compression wood direction shifts. The tree was harvested in 2008. Changes in the direction of compression wood growth (indicated 3 byChanges black arrows) in the occurred direction in the of compression years indicatedwood in thegrowth figure. (indicated by black arrows) occurred in 4 the years indicated in the figure. April and August 2008. Debris advancing every year at the 5 lobe front equals about 22 × 10 6 6 kg, or about 440 truckloads per year for a 50-ton dump truck. Analysis of an optical image time series covering the period from 1955 to 2008 shows an increase in the rate of advance of the frontal lobe of FDL-A and FDL-B (Fig. 7). During the 1955–1979 period, FDL-A moved approximately 50 m, which indicates an average rate of 2.1 m yr −1 . During the 1979–2008 period, the terminus advanced 100 m, which indicates a higher average rate of 3.4 m yr −1 . The average movement rate for the entire observation record is 2.8 m yr −1 . The current (2009–2010) movement rate we measured in the field is just over 1 cm day −1 (∼3.6 m yr −1 ), which may indicate an ongoing increase in the average movement rate. In summary, the front of FDL-A advanced 150 m since 1955, is currently less than 70 m away from the Dalton Highway, has a base cross-sectional width of 170 m, an average height of 20 m, and an accelerating terminus movement rate of 1 cm day −1 . 4.5 Debris composition Based on the geological map of the Chandalar Quadrangle by Brosgé and Reiser (1964), the bedrock immediately adjacent to the frozen debris-lobes in this reconnaissance study consists of slate, phyllite, schist, phyllitic siltstone, schistose sandstone and limestone. The frozen debris-lobes contained clasts of these rock types embedded in a complex fine grained matrix with massive and interstitial ice. We took two www.nat-hazards-earth-syst-sci.net/12/1521/2012/ Nat. Hazards Earth Syst. Sci., 12, 1521–1537, 2012 35
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